Brazing is a process for joining parts, often of dissimilar compositions, to each other. Typically, a filler metal that has a melting point lower than that of the parts to be joined together is interposed between the parts to form an assembly. The juncture of the assembled parts and the filler metal is then heated to a temperature sufficient to melt the filler metal but generally lower than the melting point of the parts. Upon cooling, a strong, void free joint is formed.
One class of products produced by brazing processes is cemented carbide cutting tools. These tools are usually formed by brazing the cemented carbide cutting tip to a holder or shank. Many of these tools are used under condition which expose them to high temperature, high stresses and high erosion conditions; for example, drill bits for oil exploration and well drilling. Accordingly, it is necessary to insure that the brazing filler metal employed has excellent high temperature strength, corrosion resistance and erosion resistance.
A particular problem, however, with cemented carbide tools is that they are prone to brittle fracture under conditions or rather moderate gradients in stress distribution, particularly those gradients created by non-uniform thermal expansion. Therefore, it is essential that the brazing filler metal employed exhibit a low melting point in order to minimize stresses induced by thermal expansion. In addition, more recently, the need for low temperature brazing metal was recognized for the production of cemented carbide tools which include polycrystalline diamond cutting tips. Specifically, in the manufacture of these diamond-tipped tools, brazing of the diamond-tipped cemented carbide cutter to the shank must be carried out at low temperatures because of the high degree of thermal instability exhibited by the polycrystalline diamond tip.
Until now, the brazing industry employed three different brazing filler materials in the production of cemented carbide parts intended to operate at elevated temperatures. The first filler metal, Au-18Ni, is a very expensive alloy which brazes well but suffers from a rather high melting temperature range (925.degree.-985.degree. C.) and from a low rupture strength at elevated temperatures. The second filler metal, Cu-10Co-31.5Mn, also brazes well but, like the gold-based alloy, has a high melting temperature range. In addition, this copper-based alloy has low corrosion resistance and poor high temperature strength because of its high manganese content. More recently, a class of brazing filler metals was developed which exhibit high temperature strength, good corrosion resistance and good erosion resistance. This third class of alloys, disclosed in U.S. Pat. Nos. 4,508,257, 4,405,391 and 4,448,618, are Ni-Pd base alloys. These alloys, however, exhibit high melting points and were originally developed for brazing superalloys. In addition, when employed to braze cemented carbide parts, they tend to react with the cemented carbide parts, leaching the cobalt from the base metal and thereby causing microporosity at the braze interface. This microporosity increases the risk of catastrophic failure in the brazed joints resulting from brittle fracture in the base metal.
There remains a need in the art for new brazing filler metal which can be used at low brazing temperatures (preferably, below about 925.degree. C.), do not react adversely with the base metal and, at the same time, exhibit good strength at elevated temperatures, good corrosion resistance and good erosion resistance.